As is known, in printing machines the transfer of ink to the material to be printed on is basically achieved via three cylinders: an anilox roller, a plate roller and an impression roller.
Specifically, the function of the anilox roller is to ink the plate, which is carried by the plate roller, which in turn prints on the material, which is carried by the impression roller.
Typically, the anilox roller has a lateral anilox surface, that is to say it is provided with a plurality of cells that are open outward, inside which the ink is deposited via an inking device, known in the printing trade as a closed chamber doctor blade.
It should be noted that, for practical reasons when changing the print format, in most flexographic printing machines the anilox roller, instead of being constituted by an integral anilox roller, is now implemented by means of a sleeve cylinder upon which an anilox sleeve can be fitted and from which the anilox sleeve can be removed which is, in practice, constituted by a tubular body provided with an outer anilox lateral surface.
In general, the inking device associated with each anilox roller goes from an inking chamber, which is open toward the lateral surface of the anilox roller and is supplied with ink under pressure so that the ink fills the cells of the anilox roller. On mutually opposite longitudinal sides of the inking chamber, two doctor blades are situated which scrape the excess ink off the cells of the anilox roller.
The friction between the doctor blades and the surface of the rotating anilox roller generates heat.
For solvent-based or water-based inks, the heat produced causes the evaporation of the solvent or of the water in sufficient measure to keep the temperature of the ink and of the anilox roller substantially at ambient temperature.
For UV and EB-type inks (that is to say, inks that can be dried using ultraviolet rays or electron beams), which, as is known, are much more viscous than traditional inks and have a low percentage of volatile substances, the heat produced causes a sharp increase in temperature, making it necessary to adopt cooling systems in order to remove the heat and prevent high ink temperatures which could compromise the print process. This is especially true of EB-type inks, due to the fact that, when these exceed a certain temperature (around 30° C.), they quickly deteriorate and are no longer usable.
In some known solutions, these cooling systems are basically constituted by a central refrigeration unit that supplies, by means of a pump, cold water under pressure, to each of the sleeve rollers.
Current cooling systems are not very effective in controlling the temperature when using sleeve rollers, because anilox sleeves have a low thermal conductivity and, therefore, it is difficult to effectively remove the heat generated by the friction of the doctor blades on the outer surface of the anilox sleeves, especially when using UV and EB inks.
Traditional anilox sleeves are, in fact, constituted by multiple layers, which, beginning at the inside of the sleeves and moving outward, are provided, respectively, by: a fiberglass inner tube of a thickness of around 1.5 mm; a layer of soft rubber, necessary for the pneumatic expansion of the fiberglass to fit the sleeve over the sleeve cylinder and remove it from the latter; an aluminum tube of around 10 mm in thickness, the outer surface of which is provided with a layer of ceramic of a few tenths of a millimeter, which is laser-cut to form the cells which are of variable sizes and shapes as a function of the inking desired.
Because some of the materials used to make anilox sleeves have a low thermal conductivity, traditional cooling systems exhibit the drawback of performing a temperature control that is considerably slow and, therefore, inadequate for the specific requirements of flexographic machines.
Moreover, also because of the low thermal conductivity of the sleeves, current cooling systems have to operate with a large difference in temperature with respect to the outer surface of the sleeve and consequently they are not very efficient in terms of energy.
Another disadvantage of current cooling systems consists in that they are capable of controlling the temperature of the anilox roller only through cooling and not through heating as well.